The land fertility is measured in kilograms of equivalent vegetable food per hectare and year. Arable land can be used to either grow vegetable food directly, or alternatively, to raise animals that then enter the human food chain. Yet it takes more land per human to raise animals than to grow vegetable food. For this reason, it is meaningful to convert proteines obtained from eating meat to equivalent units of vegetable food.
Notice that this model uses two different types of rate models. Whereas the yield technology rate is a so-called bi-flow, i.e., it can assume both positive and negative values leading to a change of flow direction, the land fertility integrator uses uni-flows, i.e., flows that are allowed to flow in the indicated direction only.